Network Working Group S. Floyd
Request for Comments: 4828 ICIR
Category: Experimental E. Kohler
UCLA
April 2007
TCP Friendly Rate Control (TFRC):The Small-Packet (SP) Variant
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document proposes a mechanism for further experimentation, but
not for widespread deployment at this time in the global Internet.
TCP-Friendly Rate Control (TFRC) is a congestion control mechanism
for unicast flows operating in a best-effort Internet environment
(RFC 3448). TFRC was intended for applications that use a fixed
packet size, and was designed to be reasonably fair when competing
for bandwidth with TCP connections using the same packet size. This
document proposes TFRC-SP, a Small-Packet (SP) variant of TFRC, that
is designed for applications that send small packets. The design
goal for TFRC-SP is to achieve the same bandwidth in bps (bits per
second) as a TCP flow using packets of up to 1500 bytes. TFRC-SP
enforces a minimum interval of 10 ms between data packets to prevent
a single flow from sending small packets arbitrarily frequently.
Flows using TFRC-SP compete reasonably fairly with large-packet TCP
and TFRC flows in environments where large-packet flows and small-
packet flows experience similar packet drop rates. However, in
environments where small-packet flows experience lower packet drop
rates than large-packet flows (e.g., with Drop-Tail queues in units
of bytes), TFRC-SP can receive considerably more than its share of
the bandwidth.
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RFC 4828 TFRC: The SP Variant April 20071. Introduction
This document specifies TFRC-SP, an experimental, Small-Packet
variant of TCP-friendly Rate Control (TFRC) [RFC3448].
TFRC was designed to be reasonably fair when competing for bandwidth
with TCP flows, but to avoid the abrupt changes in the sending rate
characteristic of TCP's congestion control mechanisms. TFRC is
intended for applications such as streaming media applications where
a relatively smooth sending rate is of importance. Conventional TFRC
measures loss rates by estimating the loss event ratio as described
in [RFC3448], and uses this loss event rate to determine the sending
rate in packets per round-trip time. This has consequences for the
rate that a TFRC flow can achieve when sharing a bottleneck with
large-packet TCP flows. In particular, a low-bandwidth, small-packet
TFRC flow sharing a bottleneck with high-bandwidth, large-packet TCP
flows may be forced to slow down, even though the TFRC application's
nominal rate in bytes per second is less than the rate achieved by
the TCP flows. Intuitively, this would be "fair" only if the network
limitation was in packets per second (such as a routing lookup),
rather than bytes per second (such as link bandwidth). Conventional
wisdom is that many of the network limitations in today's Internet
are in bytes per second, even though the network limitations of the
future might move back towards limitations in packets per second.
TFRC-SP is intended for flows that need to send frequent small
packets, with less than 1500 bytes per packet, limited by a minimum
interval between packets of 10 ms. It will better support
applications that do not want their sending rates in bytes per second
to suffer from their use of small packets. This variant is
restricted to applications that send packets no more than once every
10 ms (the Min Interval or minimum interval). Given this
restriction, TFRC-SP effectively calculates the TFRC fair rate as if
the bottleneck restriction was in bytes per second. Applications
using TFRC-SP could have a fixed or naturally-varying packet size, or
could vary their packet size in response to congestion. Applications
that are not willing to be limited by a minimum interval of 10 ms
between packets, or that want to send packets larger than 1500 bytes,
should not use TFRC-SP. However, for applications with a minimum
interval of at least 10 ms between packets and with data packets of
at most 1500 bytes, the performance of TFRC-SP should be at least as
good as that from TFRC.
RFC 3448, the protocol specification for TFRC, stated that TFRC-PS
(for TFRC-PacketSize), a variant of TFRC for applications that have a
fixed sending rate but vary their packet size in response to
congestion, would be specified in a later document. This document
instead specifies TFRC-SP, a variant of TFRC designed for
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RFC 4828 TFRC: The SP Variant April 2007
applications that send small packets, where applications could either
have a fixed or varying packet size or could adapt their packet size
in response to congestion. However, as discussed in Section 6 of
this document, there are many questions about how such an adaptive
application would use TFRC-SP that are beyond the scope of this
document, and that would need to be addressed in documents that are
more application-specific.
TFRC-SP is motivated in part by the approach in RFC 3714, which
argues that it is acceptable for VoIP flows to assume that the
network limitation is in bytes per second (Bps) rather in packets per
second (pps), and to have the same sending rate in bytes per second
as a TCP flow with 1500-byte packets and the same packet drop rate.
RFC 3714 states the following:
"While the ideal would be to have a transport protocol that is
able to detect whether the bottleneck links along the path are
limited in Bps or in pps, and to respond appropriately when the
limitation is in pps, such an ideal is hard to achieve. We would
not want to delay the deployment of congestion control for
telephony traffic until such an ideal could be accomplished. In
addition, we note that the current TCP congestion control
mechanisms are themselves not very effective in an environment
where there is a limitation along the reverse path in pps. While
the TCP mechanisms do provide an incentive to use large data
packets, TCP does not include any effective congestion control
mechanisms for the stream of small acknowledgement packets on the
reverse path. Given the arguments above, it seems acceptable to
us to assume a network limitation in Bps rather than in pps in
considering the minimum sending rate of telephony traffic."
Translating the discussion in [RFC3714] to the congestion control
mechanisms of TFRC, it seems acceptable to standardize a variant of
TFRC that allows low-bandwidth flows sending small packets to achieve
a rough fairness with TCP flows in terms of the sending rate in Bps,
rather than in terms of the sending rate in pps. This is
accomplished by TFRC-SP, a small modification to TFRC, as described
below.
Maintaining incentives for large packets: Because the bottlenecks in
the network in fact can include limitations in pps as well as in Bps,
we pay special attention to the potential dangers of encouraging a
large deployment of best-effort traffic in the Internet consisting
entirely of small packets. This is discussed in more detail in
Section 4.3. In addition, as again discussed in Section 4.3, TFRC-SP
includes the limitation of the Min Interval between packets of 10 ms.
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RFC 4828 TFRC: The SP Variant April 2007
Packet drop rates as a function of packet size: TFRC-SP essentially
assumes that the small-packet TFRC-SP flow receives roughly the same
packet drop rate as a large-packet TFRC or TCP flow. As we show,
this assumption is not necessarily correct for all environments in
the Internet.
Initializing the Loss History after the First Loss Event: Section6.3.1 of RFC 3448 specifies that the TFRC receiver initializes the
loss history after the first loss event by calculating the loss
interval that would be required to produce the receive rate measured
over the most recent round-trip time. In calculating this loss
interval, TFRC-SP uses the segment size of 1460 bytes, rather than
the actual segment size used in the connection.
Calculating the loss event rate for TFRC-SP: TFRC-SP requires a
modification in TFRC's calculation of the loss event rate, because a
TFRC-SP connection can send many small packets when a standard TFRC
or TCP connection would send a single large packet. It is not
possible for a standard TFRC or TCP connection to repeatedly send
multiple packets per round-trip time in the face of a high packet
drop rate. As a result, TCP and standard TFRC only respond to a
single loss event per round-trip time, and are still able to detect
and respond to increasingly heavy packet loss rates. However, in a
highly-congested environment, when a TCP connection might be sending,
on average, one large packet per round-trip time, a corresponding
TFRC-SP connection might be sending many small packets per round-trip
time. As a result, in order to maintain fairness with TCP, and to be
able to detect changes in the degree of congestion, TFRC-SP needs to
be sensitive to the actual packet drop rate during periods of high
congestion. This is discussed in more detail in the section below.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. TFRC-SP Congestion Control
TFRC uses the TCP throughput equation given in Section 3.1 of
[RFC3448], which gives the allowed sending rate X in bytes per second
as a function of the loss event rate, segment size, and round-trip
time. [RFC3448] specifies that the segment size s used in the
throughput equation should be the segment size used by the
application, or the estimated mean segment size if there are
variations in the segment size depending on the data. This gives
rough fairness with TCP flows using the same segment size.
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RFC 4828 TFRC: The SP Variant April 2007
TFRC-SP changes this behavior in the following ways.
o The nominal segment size: The nominal segment size s defaults to
1460 bytes. Following [RFC3714], this provides a goal of
fairness, in terms of the sending rate in bytes per second, with a
TCP flow with 1460 bytes of application data per packet but with
the same packet drop rate. If the endpoint knows the MTU (Maximum
Transmission Unit) of the path and the derived MSS (Maximum
Segment Size) is less than 1460 bytes, then the endpoint SHOULD
set the nominal segment size s to MSS bytes. In addition, if the
endpoint knows the MTU of the path and the resulting MSS is less
than 536 bytes, then the endpoint MUST set the nominal segment
size s to MSS bytes.
However, this document does not require that TFRC-SP endpoints
determine the path MTU. While most paths allow an MSS of 1460
bytes, some paths have a slightly smaller MSS due to tunnels
(e.g., IPv6 over IPv4). In some specific cases, IPv4 paths may
experience a much smaller path MTU. Due to the complications of
estimating the path MTU, and to the fact that most paths support
an MSS of at least 536 bytes, TFRC-SP as a default uses a nominal
segment size of 1460 bytes. The nominal segment size is discussed
in more detail in Section 4.5.3.
o Taking packet headers into account: The allowed transmit rate X in
bytes per second is reduced by a factor that accounts for packet
header size. This gives the application some incentive, beyond
the Min Interval, not to use unnecessarily small packets. In
particular, we introduce a new parameter H, which represents the
expected size in bytes of network and transport headers to be used
on the TFRC connection's packets. This is used to reduce the
allowed transmit rate X as follows:
X := X * s_true / (s_true + H),
where s_true is the true average data segment size for the
connection in bytes, excluding the transport and network headers.
Section 4.1 of RFC 3448 states that where the packet size varies
naturally with the data, an estimate of the mean segment size can
be used for s_true. As suggested in Section 4.1 of [RFC3448bis],
when an estimate of the mean segment size is used for s_true, the
estimate SHOULD be calculated over at least the last four loss
intervals. However, this document does not specify a specific
algorithm for estimating the mean segment size.
The H parameter is set to the constant 40 bytes. Thus, if the
TFRC-SP application used 40-byte data segments, the allowed
transmit rate X would be halved to account for the fact that half
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RFC 4828 TFRC: The SP Variant April 2007
of the sending rate would be used by the headers. Section 4.2
justifies this definition. However, a connection using TFRC-SP
MAY instead use a more precise estimate of H, based on the actual
network and transport headers to be used on the connection's
packets. For example, a Datagram Congestion Control Protocol
(DCCP) connection [RFC4340] over IPv4, where data packets use the
DCCP-Data packet type, and there are no IP or DCCP options, could
set H to 20 + 12 = 32 bytes. However, if the TFRC implementation
knows that the IP layer is using IPv6 instead of IPv4, then the
connection using TFRC-SP MAY still use the default estimate of 40
bytes for H instead of the actual size of 60 bytes, for simplicity
of implementation.
o Measuring the loss event rate in times of high loss: During short
loss intervals (those at most two round-trip times), the loss rate
is computed by counting the actual number of packets lost or
marked, not by counting at most one loss event per loss interval.
Without this change, TFRC-SP could send multiple packets per
round-trip time even in the face of heavy congestion, for a
steady-state behavior of multiple packets dropped each round-trip
time.
In standard TFRC, the TFRC receiver estimates the loss event rate
by calculating the average loss interval in packets, and inverting
to get the loss event rate. Thus, for a short loss interval with
N packets and K losses, standard TFRC calculates the size of that
loss interval as N packets, contributing to a loss event rate of
1/N. However, for TFRC-SP, for small loss intervals of at most
two round-trip times, if the loss interval consists of N packets
including K losses, the size of the loss interval is calculated as
N/K, contributing to a loss event rate of K/N instead of 1/N.
Section 5.4 of RFC 3448 specifies that the calculation of the
average loss interval includes the most recent loss interval only
if this increases the calculated average loss interval, as in the
pseudocode below. However, in TFRC-SP the calculated loss
interval size for a short loss interval varies as a function of
the number of packet losses that have been detected, allowing
either increases or decreases in the calculated loss interval size
for the current short loss interval as new packets are received.
Therefore, TFRC-SP adds the restriction that the calculation of
the average loss interval can include the most recent loss
interval only if more than two round-trip times have passed since
the beginning of that loss interval.
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RFC 4828 TFRC: The SP Variant April 20074. TFRC-SP Discussion4.1. Response Functions and Throughput Equations
TFRC uses the TCP throughput equation given in [RFC3448], with the
sending rate X in bytes per second as follows:
s
X = ------------------------------------------------------- ,
R*sqrt(2*p/3) + (4*R* (3*sqrt(3*p/8) * p * (1+32*p^2)))
where:
s is the packet size in bytes;
R is the round-trip time in seconds;
p is the loss event rate, between 0 and 1.0, of the number of loss
events as a fraction of the number of packets transmitted.
This equation uses a retransmission timeout (RTO) of 4*R, and assumes
that the TCP connection sends an acknowledgement for every data
packet.
This equation essentially gives the response function for TCP as well
as for standard TFRC (modulo TCP's range of sender algorithms for
setting the RTO). As shown in Table 1 of [RFC3714], for high packet
drop rates, this throughput equation gives rough fairness with the
most aggressive possible current TCP: a SACK TCP flow using
timestamps and Explicit Congestion Notification (ECN). Because it is
not recommended for routers to use ECN-marking in highly-congested
environments with high packet dropping/marking rates (Section 7 of
[RFC3168]), we note that it would be useful to have a throughput
equation with a somewhat more moderate sending rate for packet drop
rates of 40% and above.
The effective response function of TFRC-SP can be derived from the
TFRC response function by using a segment size s of 1460 bytes, and
using the loss event rate actually experienced by the TFRC-SP flow.
In addition, for loss intervals of at most two round-trip times, the
loss event rate for TFRC-SP is estimated by counting the actual
number of lost or marked packets, rather than by counting loss
events. In addition, the sending rate for TFRC-SP is constrained to
be at most 100 packets per second.
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RFC 4828 TFRC: The SP Variant April 2007
<---------- TFRC-SP -------->
Byte 14-byte 536-byte 1460-byte
DropRate Segments Segments Segments
-------- -------- -------- --------
0.0000001 5.40 57.60 150.00
0.0000003 5.40 57.60 150.00
0.0000010 5.40 57.60 150.00
0.0000030 5.40 57.60 150.00
0.0000100 5.40 57.60 132.75
0.0000300 5.40 57.60 61.70
0.0001000 5.40 50.00 16.25
0.0003000 5.40 12.89 1.70
0.0010000 5.40 0.95 0.15
0.0030000 5.40 0.12 0.06
0.0100000 1.10 0.06 0.06
0.0300000 0.13 0.06 0.06
Table 4: Response Function for TFRC-SP.
Sending Rate in KBps, as a Function of Byte Drop Rate.
Maximum Sending Rate of 100 Packets per Second.
For Tables 3 and 4, the packet drop rate is calculated as 1-(1-b)^N,
for a byte drop rate of b, and a packet size of N bytes. These
tables use the packet loss rate as an approximation for the loss
event rate p. The TCP response functions shown in Table 3 for fixed
byte drop rates are rather different from the response functions
shown in Table 1 for fixed packet drop rates; with higher byte drop
rates, a TCP connection can have a higher sending rate using
*smaller* packets. Table 4 also shows that with fixed byte drop
rates, the sending rate for TFRC-SP can be significantly higher than
that for TCP or standard TFRC, regardless of the TCP segment size.
This is because in this environment, with small packets, TFRC-SP
receives a small packet drop rate, but is allowed to send at the
sending rate of a TCP or standard TFRC flow using larger packets but
receiving the same packet drop rate.
Simulations showing TCP, standard TFRC, and TFRC-SP sending rates in
response to a configured byte drop rate are given in Appendix B.2.
4.2. Accounting for Header Size
[RFC3714] makes the optimistic assumption that the limitation of the
network is in bandwidth in bytes per second (Bps), and not in CPU
cycles or in packets per second (pps). However, some attention must
be paid to the load in pps as well as to the load in Bps. Even aside
from the Min Interval, TFRC-SP gives the application some incentive
to use fewer but larger packets, when larger packets would suffice,
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RFC 4828 TFRC: The SP Variant April 2007
by including the bandwidth used by the packet header in the allowed
sending rate.
As an example, a sender using 120-byte packets needs a TCP-friendly
rate of 128 Kbps to send 96 Kbps of application data. This is
because the TCP-friendly rate is reduced by a factor of
s_true/(s_true + H) = 120/160, to account for the effect of packet
headers. If the sender suddenly switched to 40-byte data segments,
the allowed sending rate would reduce to 64 Kbps of application data;
and the use of one-byte data segments would reduce the allowed
sending rate to 3.12 Kbps of application data. (In fact, the Min
Interval would prevent senders from achieving these rates, since
applications using TFRC-SP cannot send more than 100 packets per
second.)
Unless it has a more precise estimate of the header size, TFRC-SP
assumes 40 bytes for the header size, although the header could be
larger (due to IP options, IPv6, IP tunnels, and the like) or smaller
(due to header compression) on the wire. Requiring the use of the
exact header size in bytes would require significant additional
complexity, and would have little additional benefit. TFRC-SP's
default assumption of a 40-byte header is sufficient to get a rough
estimate of the throughput, and to give the application some
incentive not to use an excessive amount of small packets. Because
we are only aiming at rough fairness, and at a rough incentive for
applications, the default use of a 40-byte header in the calculations
of the header bandwidth is sufficient for both IPv4 and IPv6.
4.3. The TFRC-SP Min Interval
The header size calculation provides an incentive for applications to
use fewer, but larger, packets. Another incentive is that when the
path limitation is in pps, the application using more small packets
is likely to cause higher packet drop rates, and to have to reduce
its sending rate accordingly. That is, if the congestion is in terms
of pps, then the flow sending more pps will increase the packet drop
rate, and have to adjust its sending rate accordingly. However, the
increased congestion caused by the use of small packets in an
environment limited by pps is experienced not only by the flow using
the small packets, but by all of the competing traffic on that
congested link. These incentives are therefore insufficient to
provide sufficient protection for pps network limitations.
TFRC-SP, then, includes a Min Interval of 10 ms. This provides
additional restrictions on the amount of small packets used.
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RFC 4828 TFRC: The SP Variant April 2007
One practical justification for the Min Interval is that the
applications that currently want to send small packets are the VoIP
applications that send at most one packet every 10 ms, so this
restriction does not affect current traffic. A second justification
is that there is no pressing need for best-effort traffic in the
current Internet to send small packets more frequently than once
every 10 ms (rather than taking the 10 ms delay at the sender, and
merging the two small packets into one larger one). This 10 ms delay
for merging small packets is likely to be dominated by the network
propagation, transmission, and queueing delays of best-effort traffic
in the current Internet. As a result, our judgment would be that the
benefit to the user of having less than 10 ms between packets is
outweighed by the benefit to the network of avoiding an excessive
amount of small packets.
The Min Interval causes TFRC-SP not to support applications sending
small packets very frequently. Consider a TFRC flow with a fixed
packet size of 100 bytes, but with a variable sending rate and a
fairly uncongested path. When this flow is sending at most 100 pps,
it would be able to use TFRC-SP. If the flow wishes to increase its
sending rate to more than 100 pps, but keep the same packet size, it
would no longer be able to achieve this with TFRC-SP, and would have
to switch to the default TFRC, receiving a dramatic, discontinuous
decrease in its allowed sending rate. This seems not only
acceptable, but desirable for the global Internet.
What is to prevent flows from opening multiple connections, each with
a 10 ms Min Interval, thereby getting around the limitation of the
Min Interval? Obviously, there is nothing to prevent flows from
doing this, just as there is currently nothing to prevent flows from
using UDP, or from opening multiple parallel TCP connections, or from
using their own congestion control mechanism. Of course,
implementations or middleboxes are also free to limit the number of
parallel TFRC connections opened to the same destination in times of
congestion, if that seems desirable. And flows that open multiple
parallel connections are subject to the inconveniences of reordering
and the like.
4.4. Counting Packet Losses
It is not possible for a TCP connection to persistently send multiple
packets per round-trip time in the face of high congestion, with a
steady-state with multiple packets dropped per round-trip time. For
TCP, when one or more packets are dropped each round-trip time, the
sending rate is quickly dropped to less than one packet per round-
trip time. In addition, for TCP with Tahoe, NewReno, or SACK
congestion control mechanisms, the response to congestion is largely
independent of the number of packets dropped per round-trip time.
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RFC 4828 TFRC: The SP Variant April 2007
As a result, standard TFRC can best achieve fairness with TCP, even
in highly congested environments, by calculating the loss event rate
rather than the packet drop rate, where a loss event is one or more
packets dropped or marked from a window of data.
However, with TFRC-SP, it is no longer possible to achieve fairness
with TCP or with standard TFRC by counting only the loss event rate
[WBL04]. Instead of sending one large packet per round-trip time,
TFRC-SP could be sending N small packets (where N small packets equal
one large 1500-byte packet). The loss measurement used with TFRC-SP
has to be able to detect a connection that is consistently receiving
multiple packet losses or marks per round-trip time, to allow TFRC-SP
to respond appropriately.
In TFRC-SP, the loss event rate is calculated by counting at most one
loss event in loss intervals longer than two round-trip times, and by
counting each packet lost or marked in shorter loss intervals. In
particular, for a short loss interval with N packets, including K
lost or marked packets, the loss interval length is calculated as
N/K, instead of as N. The average loss interval I_mean is still
averaged over the eight most recent loss intervals, as specified in
Section 5.4 of RFC 3448. Thus, if eight successive loss intervals
are short loss intervals with N packets and K losses, the loss event
rate is calculated as K/N, rather than as 1/N.
4.5. The Nominal Packet Size4.5.1. Packet Size and Packet Drop Rates
The guidelines in Section 3 above say that the nominal segment size s
is set to 1460 bytes, providing a goal of fairness, in terms of the
sending rate in bytes per second, with a TCP flow with 1460 bytes of
application data per packet but with the same packet drop rate. This
follows the assumption that a TCP flow with 1460-byte segments will
have a higher sending rate than a TCP flow with smaller segments.
While this assumption holds in an environment where the packet drop
rate is independent of packet size, this assumption does not
necessarily hold in an environment where larger packets are more
likely to be dropped than are small packets.
The table below shows the results of simulations with standard (SACK)
TCP flows, where, for each *byte*, the packet containing that byte is
dropped with probability p. Consider the approximation for the TCP
response function for packet drop rates up to 10% or so; for these
environments, the sending rate in bytes per RTT is roughly
1.2 s/sqrt(p), for a packet size of s bytes and packet drop rate p.
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RFC 4828 TFRC: The SP Variant April 2007
Given a fixed *byte* drop rate p1, and a TCP packet size of s bytes,
the packet drop rate is roughly s*p1, producing a sending rate in
bytes per RTT of roughly 1.2 sqrt(s)/sqrt(p1). Thus, for TCP in an
environment with a fixed byte drop rate, the sending rate should grow
roughly as sqrt(s), for packet drop rates up to 10% or so.
Each row of Table 5 below shows a separate simulation with ten TCP
connections and a fixed byte drop rate of 0.0001, with each
simulation using a different segment size. For each simulation, the
TCP sending rate and goodput are averaged over the ten flows. As one
would expect from the paragraph above, the TCP sending rate grows
somewhat less than linearly with an increase in packet size, up to a
packet size of 1460 bytes, corresponding to a packet drop rate of
13%. After that, further increases in the packet size result in a
*decrease* in the TCP sending rate, as the TCP connection enters the
regime of exponential backoff of the retransmit timer.
Segment Packet TCP Rates (Kbps)
Size (B) DropRate SendRate Goodput
-------- -------- -------- -------
14 0.005 6.37 6.34
128 0.016 30.78 30.30
256 0.028 46.54 44.96
512 0.053 62.43 58.37
1460 0.134 94.15 80.02
4000 0.324 35.20 21.44
8000 0.531 15.36 5.76
Table 5: TCP Median Send Rate vs. Packet Size I:
Byte Drop Rate 0.0001
Table 6 below shows similar results for a byte drop rate of 0.001.
In this case, the TCP sending rate grows with increasing packet size
up to a packet size of 128 bytes, corresponding to a packet drop rate
of 16%. After that, the TCP sending rate decreases and then
increases again, as the TCP connection enters the regime of
exponential backoff of the retransmit timer. Note that with this
byte drop rate, with packet sizes of 4000 and 8000 bytes, the TCP
sending rate increases but the TCP goodput rate remains essentially
zero. This makes sense, as almost all packets that are sent are
dropped.
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RFC 4828 TFRC: The SP Variant April 2007
Segment Packet TCP Rates (Kbps)
Size (B) DropRate SendRate Goodput
-------- -------- -------- -------
14 0.053 1.68 1.56
128 0.159 7.66 6.13
256 0.248 6.21 4.32
512 0.402 1.84 1.11
1460 0.712 1.87 0.47
4000 0.870 3.20 0.00
8000 0.890 5.76 0.00
Table 6: TCP Median Send Rate vs. Packet Size II:
Byte Drop Rate 0.001
The TCP behavior in the presence of a fixed byte drop rate suggests
that instead of the goal of a TFRC-SP flow achieving the same sending
rate in bytes per second as a TCP flow using 1500-byte packets, it
makes more sense to consider an ideal goal of a TFRC-SP flow
achieving the same sending rate as a TCP flow with the optimal packet
size, given that the packet size is at most 1500 bytes. This does
not mean that we need to change the TFRC-SP mechanisms for computing
the allowed transmit rate; this means simply that in evaluating the
fairness of TFRC-SP, we should consider fairness relative to the TCP
flow using the optimal packet size (though still at most 1500 bytes)
for that environment.
4.5.2. Fragmentation and the Path MTU
This document doesn't specify TFRC-SP behavior in terms of packet
fragmentation and Path MTU Discovery (PMTUD). That is, should the
transport protocol using TFRC-SP use PMTUD information to set an
upper bound on the segment size? Should the transport protocol allow
packets to be fragmented in the network? We leave these as questions
for the transport protocol. As an example, we note that DCCP
requires that endpoints keep track of the current PMTU, and says that
fragmentation should not be the default (Section 14 of [RFC4340]).
4.5.3. The Nominal Segment Size and the Path MTU
When TFRC-SP is used with a nominal segment size s of 1460 bytes on a
path where the TCP MSS is in fact only 536 bytes, the TFRC-SP flow
could receive almost three times the bandwidth, in bytes per second,
as that of a TCP flow using an MSS of 536 bytes. Similarly, in an
environment with an MSS close to 4000 bytes, a TCP flow could receive
almost three times the bandwidth of a TFRC-SP flow with its nominal
segment size s of 1460 bytes. In both cases, we feel that these
levels of "unfairness" with factors of two or three are acceptable;
in particular, they won't result in one flow grabbing all of the
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RFC 4828 TFRC: The SP Variant April 2007
available bandwidth, to the exclusion of the competing TCP or TFRC-SP
flow.
All IPv4 *end hosts* are required to accept and reassemble IP packets
of size 576 bytes [RFC791], but IPv4 *links* do not necessarily have
to support this packet size. In slow networks, the largest possible
packet may take a considerable amount of time to send [RFC3819], and
a smaller MTU may be desirable, e.g., hundreds of bytes. If the
first-hop link had a small MTU, then TCP would choose an
appropriately small MSS [RFC879]. [RFC1144] quotes cases of very low
link speeds where the MSS may be tens of bytes (and notes this is an
extreme case). We note that if TFRC-SP is used over a path with an
MTU considerably smaller than 576 bytes, and the TFRC-SP flow uses a
nominal segment size s of 1460 bytes, then the TFRC-SP flow could
receive considerably more than three times the bandwidth of competing
TCP flows.
If TFRC-SP is used with a nominal segment size s of less than 536
bytes (because the path MTU is known to be less than 576 bytes), then
TFRC-SP is likely to be of minimal benefit to applications. If
TFRC-SP was to be used on paths that have a path MTU of considerably
less than 576 bytes, and the transport protocol was not required to
keep track of the path MTU, this could result in the TFRC-SP flow
using the default nominal segment size of 1460 bytes, and as a result
receiving considerably more bandwidth than competing TCP flows. As a
result, TFRC-SP is not recommended for use with transport protocols
that don't maintain some knowledge of the path MTU.
4.6. The Loss Interval Length for Short Loss Intervals
For a TFRC-SP receiver, the guidelines from Section 6 of RFC 3448
govern when the receiver should send feedback messages. In
particular, from [RFC3448], "a feedback packet should ... be sent
whenever a new loss event is detected without waiting for the end of
an RTT". In addition, feedback packets are sent at least once per
RTT.
For a TFRC-SP connection with a short current loss interval (less
than two round-trip times), it is possible for the loss interval
length calculated for that loss interval to change in odd ways as
additional packet losses in that loss interval are detected. To
prevent unnecessary oscillations in the average loss interval,
Section 3 specifies that the current loss interval can be included in
the calculation of the average loss interval only if the current loss
interval is longer than two round-trip times.
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RFC 4828 TFRC: The SP Variant April 20075. A Comparison with RFC 3714RFC 3714 considers the problems of fairness, potential congestion
collapse, and poor user quality that could occur with the deployment
of non-congestion-controlled IP telephony over congested best-effort
networks. The March 2004 document cites ongoing efforts in the IETF,
including work on TFRC and DCCP. RFC 3714 recommends that for best-
effort traffic with applications that have a fixed or minimum sending
rate, the application or transport protocol should monitor the packet
drop rate, and discontinue sending for a period if the steady-state
packet drop rate significantly exceeds the allowed threshold for that
minimum sending rate.
In determining the allowed packet drop rate for a fixed sending rate,
RFC 3714 assumes that an IP telephony flow should be allowed to use
the same sending rate in bytes per second as a 1500-byte packet TCP
flow experiencing the same packet drop rate as that of the IP
telephony flow. As an example, following this guideline, a VoIP
connection with a round-trip time of 0.1 sec and a minimum sending
rate of 64 Kbps would be required to terminate or suspend when the
persistent packet drop rate significantly exceeded 25%.
One limitation of the lack of fine-grained control in the minimal
mechanism described in RFC 3714 is that an IP telephony flow would
not adapt its sending rate in response to congestion. In contrast,
with TFRC-SP, a small-packet flow would reduce its sending rate
somewhat in response to moderate packet drop rates, possibly avoiding
a period with unnecessarily-heavy packet drop rates.
Because RFC 3714 assumes that the allowed packet drop rate for an IP
telephony flow is determined by the sending rate that a TCP flow
would use *with the same packet drop rate*, the minimal mechanism in
RFC 3714 would not provide fairness between TCP and IP telephony
traffic in an environment where small packets are less likely to be
dropped than large packets. In such an environment, the small-
packet IP telephony flow would make the optimistic assumption that a
large-packet TCP flow would receive the same packet drop rate as the
IP telephony flow, and as a result the small-packet IP telephony flow
would receive a larger fraction of the link bandwidth than a
competing large-packet TCP flow.
6. TFRC-SP with Applications that Modify the Packet Size
One possible use for TFRC-SP would be with applications that maintain
a fixed sending rate in packets per second, but modify their packet
size in response to congestion. TFRC-SP monitors the connection's
packet drop rate, and determines the allowed sending rate in bytes
per second. Given an application with a fixed sending rate in
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RFC 4828 TFRC: The SP Variant April 2007
packets per second, the TFRC-SP sender could determine the data
packet size that would be needed for the sending rate in bytes per
second not to exceed the allowed sending rate. In environments where
the packet drop rate is affected by the packet size, decreasing the
packet size could also result in a decrease in the packet drop rate
experienced by the flow.
There are many questions about how an adaptive application would use
TFRC-SP that are beyond the scope of this document. In particular,
an application might wish to avoid unnecessary reductions in the
packet size. In this case, an application might wait for some period
of time before reducing the packet size, to avoid an unnecessary
reduction in the packet size. The details of how long an application
might wait before reducing the packet size can be addressed in
documents that are more application-specific.
Similarly, an application using TFRC-SP might only have a few packet
sizes that it is able to use. In this case, the application might
not reduce the packet size until the current packet drop rate has
significantly exceeded the packet drop rate threshold for the current
sending rate, to avoid unnecessary oscillations between two packet
sizes and two sending rates. Again, the details will have to be
addressed in documents that are more application-specific.
Because the allowed sending rate in TFRC-SP is in bytes per second
rather than in packets per second, there is little opportunity for
applications to manipulate the packet size in order to "game" the
system. This differs from TFRC in CCID 3 (Section 5.3 of [RFC4342]),
where the allowed sending rate is in packets per second. In
particular, a TFRC-SP application that sends small packets for a
while, building up a fast sending rate, and then switches to large
packets, would not increase its overall sending rate by the change of
packet size.
7. Simulations
This section describes the performance of TFRC-SP in simulation
scenarios with configured packet or byte drop rates, and in scenarios
with a range of queue management mechanisms at the congested link.
The simulations, described in detail in Appendix B, explore
environments where standard TFRC significantly limits the throughput
of small-packet flows, and TFRC-SP gives the desired throughput. The
simulations also explore environments where standard TFRC allows
small-packet flows to receive good performance, while TFRC-SP is
overly aggressive.
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The general lessons from the simulations are as follows.
o In scenarios where large and small packets receive similar packet
drop rates, TFRC-SP gives roughly the desired sending rate
(Appendix B.1, B.2).
o In scenarios where each *byte* is equally likely to be dropped,
standard TFRC gives reasonable fairness between small-packet TFRC
flows and large-packet TCP flows (Appendix B.2).
o In scenarios where small packets are less likely to be dropped
than large packets, TFRC-SP does not give reasonable fairness
between small-packet TFRC-SP flows and large-packet TCP flows;
small-packet TFRC-SP flows can receive considerably more bandwidth
than competing large-packet TCP flows, and in some cases large-
packet TCP flows can be essentially starved by competing small-
packet TFRC-SP flows (Appendix B.2, B.3, B.4).
o Scenarios where small packets are less likely to be dropped than
large packets include those with Drop-Tail queues in bytes, and
with AQM mechanisms in byte mode (Appendix B.3, B.4). It has also
been reported that wireless links are sometimes good enough to let
small packets through, while larger packets have a much higher
error rate, and hence a higher drop probability [S05].
8. General Discussion
Dropping rates for small packets: The goal of TFRC-SP is for small-
packet TFRC-SP flows to have rough fairness with large-packet TCP
flows in the sending rate in bps, in a scenario where each packet
receives roughly the same probability of being dropped. In a
scenario where large packets are more likely to be dropped than small
packets, or where flows with a bursty sending rate are more likely to
have packets dropped than are flows with a smooth sending rate,
small-packet TFRC-SP flows can receive significantly more bandwidth
than competing large-packet TCP flows.
The accuracy of the TCP response function used in TFRC: For
applications with a maximum sending rate of 96 Kbps or less (i.e.,
packets of at most 120 bytes), TFRC-SP only restricts the sending
rate when the packet drop rate is fairly high, e.g., greater than
10%. [Derivation: A TFRC-SP flow with a 200 ms round-trip time and a
maximum sending rate with packet headers of 128 Kbps would have a
sending rate in bytes per second equivalent to a TCP flow with 1460-
byte segments sending 2.2 packets per round-trip time. From Table 1
of RFC 3714, this sending rate can be sustained with a packet drop
rate slightly greater than 10%.] In this high-packet-drop regime,
the performance of TFRC-SP is determined in part by the accuracy of
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RFC 4828 TFRC: The SP Variant April 2007
the TCP response function in representing the actual sending rate of
a TCP connection.
In the regime of high packet drop rates, TCP performance is also
affected by the TCP algorithm (e.g., SACK or not), the minimum RTO,
the use of Limited Transmit (or lack thereof), the use of ECN, and
the like. It is good to ensure that simulations or experiments
exploring fairness include the exploration of fairness with the most
aggressive TCP mechanisms conformant with the current standards. Our
simulations use SACK TCP with Limited Transmit and with a minimum RTO
of 200 ms. The simulation results are largely the same with or
without timestamps; timestamps were not used for simulations reported
in this paper. We didn't use TCP with ECN in setting the target
sending rate for TFRC, because, as explained in Appendix B.1, our
expectation is that in high packet drop regimes, routers will drop
rather than mark packets, either from policy or from buffer overflow.
Fairness with different packet header sizes: In an environment with
IPv6 and/or several layers of network-layer tunnels (e.g., IPsec,
Generic Routing Encapsulation (GRE)), the packet header could be 60,
80, or 100 bytes instead of the default 40 bytes assumed in Section3. For an application with small ten-byte data segments, this means
that the actual packet size could be 70, 90, or 110 bytes, instead of
the 50 bytes assumed by TFRC-SP in calculating the allowed sending
rate. Thus, a TFRC-SP application with large headers could receive
more than twice the bandwidth (including the bandwidth used by
headers) than a TFRC-SP application with small headers. We do not
expect this to be a problem; in particular, TFRC-SP applications with
large headers will not significantly degrade the performance of
competing TCP applications, or of competing TFRC-SP applications with
small headers.
General issues for TFRC: The congestion control mechanisms in TFRC
and TFRC-SP limit a flow's sending rate in packets per second.
Simulations by Tom Phelan [P04] explore how such a limitation in
sending rate can lead to unfairness for the TFRC flow in some
scenarios, e.g., when competing with bursty TCP web traffic, in
scenarios with low levels of statistical multiplexing at the
congested link.
9. Security Considerations
There are no new security considerations introduced in this document.
The issues of the fairness of TFRC-SP with standard TFRC and TCP in a
range of environments, including those with byte-based queue
management at the congested routers, are discussed extensively in the
main body of this document.
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General security considerations for TFRC are discussed in RFC 3448.
As with TFRC in RFC 3448, TFRC-SP is not a transport protocol in its
own right, but a congestion control mechanism that is intended to be
used in conjunction with a transport protocol. Therefore, security
primarily needs to be considered in the context of a specific
transport protocol and its authentication mechanisms. As discussed
for TFRC in RFC 3448, any transport protocol that uses TFRC-SP needs
to protect against spoofed feedback, and to protect the congestion
control mechanisms against incorrect information from the receiver.
Again as discussed for TFRC in RFC 3448, we expect that protocols
incorporating ECN with TFRC-SP will want to use the ECN nonce
[RFC3540] to protect the sender from the accidental or malicious
concealment of marked packets
Security considerations for DCCP's Congestion Control ID 3, TFRC
Congestion Control, the transport protocol that uses TFRC, are
discussed in [RFC4342]. That document extensively discussed the
mechanisms the sender can use to verify the information sent by the
receiver, including the use of the ECN nonce.
10. Conclusions
This document has specified TFRC-SP, a Small-Packet (SP) variant of
TFRC, designed for applications that send small packets, with at most
a hundred packets per second, but that desire the same sending rate
in bps as a TCP connection experiencing the same packet drop rate but
sending packets of 1500 bytes. TFRC-SP competes reasonably well with
large-packet TCP and TFRC flows in environments where large-packet
flows and small-packet flows experience similar packet drop rates,
but receives more than its share of the bandwidth in bps in
environments where small packets are less likely to be dropped or
marked than are large packets. As a result, TFRC-SP is experimental,
and is not intended for widespread deployment at this time in the
global Internet.
In order to allow experimentation with TFRC-SP in the Datagram
Congestion Control Protocol (DCCP), an experimental Congestion
Control IDentifier (CCID) will be used, based on TFRC-SP but
specified in a separate document.
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RFC 4828 TFRC: The SP Variant April 200711. Acknowledgements
We thank Tom Phelan for discussions of TFRC-SP and for his paper
exploring the fairness between TCP and TFRC-SP flows. We thank Lars
Eggert, Gorry Fairhurst, Mark Handley, Miguel Garcia, Ladan Gharai,
Richard Nelson, Colin Perkins, Juergen Quittek, Pete Sholander,
Magnus Westerlund, and Joerg Widmer for feedback on earlier versions
of this document. We also thank the DCCP Working Group for feedback
and discussions.
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RFC 4828 TFRC: The SP Variant April 2007Appendix A. Related Work on Small-Packet Variants of TFRC
Other proposals for variants of TFRC for applications with variable
packet sizes include [WBL04] and [V00]. [V00] proposed that TFRC
should be modified so that flows are not penalized by sending smaller
packets. In particular, [V00] proposes using the path MTU in the
TCP-friendly equation, instead of the actual packet size used by
TFRC, and counting the packet drop rate by using an estimation
algorithm that aggregates both packet drops and received packets into
MTU-sized units.
[WBL04] also argues that adapting TFRC for variable packet sizes by
just using the packet size of a reference TCP flow in the TFRC
equation would not suffice in the high-packet-loss regime; such a
modified TFRC would have a strong bias in favor of smaller packets,
because multiple lost packets in a single round-trip time would be
aggregated into a single packet loss. [WBL04] proposes modifying the
loss measurement process to account for the bias in favor of smaller
packets.
The TFRC-SP variant of TFRC proposed in our document differs from
[WBL04] in restricting its attention to flows that send at most 100
packets per second. The TFRC-SP variant proposed in our document
also differs from the straw proposal discussed in [WBL04] in that the
allowed bandwidth includes the bandwidth used by packet headers.
[WBL04] discusses four methods for modifying the loss measurement
process, "unbiasing", "virtual packets", "random sampling", and "Loss
Insensitive Period (LIP) scaling". [WBL04] finds only the second and
third methods sufficiently robust when the network drops packets
independently of packet size. They find only the second method
sufficiently robust when the network is more likely to drop large
packets than small packets. Our method for calculating the loss
event rate is somewhat similar to the random sampling method proposed
in [WBL04], except that randomization is not used; instead of
randomization, the exact packet loss rate is computed for short loss
intervals, and the standard loss event rate calculation is used for
longer loss intervals. [WBL04] includes simulations with a Bernoulli
loss model, a Bernoulli loss model with a drop rate varying over
time, and a Gilbert loss model, as well as more realistic simulations
with a range of TCP and TFRC flows.
[WBL04] produces both a byte-mode and a packet-mode variant of the
TFRC transport protocol, for connections over paths with per-byte and
per-packet dropping respectively. We would argue that in the absence
of transport-level mechanisms for determining whether the packet
dropping in the network is per-packet, per-byte, or somewhere in
between, a single TFRC implementation is needed, independently of the
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RFC 4828 TFRC: The SP Variant April 2007
packet-dropping behaviors of the routers along the path. It would of
course be preferable to have a mechanism that gives roughly
acceptable behavior, to the connection and to the network as a whole,
on paths with both per-byte and per-packet dropping (and on paths
with multiple congested routers, some with per-byte dropping
mechanisms, some with per-packet dropping mechanisms, and some with
dropping mechanisms that lie somewhere between per-byte and per-
packet).
An important contribution would be to investigate the range of
behaviors actually present in today's networks, in terms of packet
dropping as a function of packet size.
Appendix B. Simulation Results
This appendix reports on the simulation results outlined in
Section 7. TFRC-SP has been added to the NS simulator, and is
illustrated in the validation test "./test-all-friendly" in the
directory tcl/tests. The simulation scripts and graphs for the
simulations in this document are available at [VOIPSIMS].
B.1. Simulations with Configured Packet Drop Rates
In this section we describe simulation results from simulations
comparing the throughput of standard (SACK) TCP flows, TCP flows with
timestamps and ECN, TFRC-SP flows, and standard TFRC (Stnd TFRC)
flows. In these simulations we configure the router to randomly drop
or mark packets at a specified rate, independently of the packet
size. For each specified packet drop rate, we give a flow's average
sending rate in Kbps over the second half of a 100-second simulation,
averaged over ten flows.
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RFC 4828 TFRC: The SP Variant April 2007
Packet Send Rates (Kbps, 1460B seg)
DropRate TCP ECN TCP TFRC
-------- -------- -------- --------
0.001 2020.85 1904.61 982.09
0.005 811.10 792.11 878.08
0.01 515.45 533.19 598.90
0.02 362.93 382.67 431.41
0.04 250.06 252.64 284.82
0.05 204.48 218.16 268.51
0.1 143.30 148.41 146.03
0.2 78.65 93.23* 55.14
0.3 26.26 59.65* 32.87
0.4 9.87 47.79* 25.45
0.5 3.53 32.01* 18.52
* ECN scenarios marked with an asterisk are not realistic,
as routers are not recommended to mark packets when packet
drop/mark rates are 20% or higher.
Table 7: Send Rate vs. Packet Drop Rate I:
1460B TFRC Segments
(1.184 Kbps Maximum TFRC Data Sending Rate)
Table 7 shows the sending rate for a TCP and a standard TFRC flow for
a range of configured packet drop rates, when both flows have 1460-
byte data segments, in order to illustrate the relative fairness of
TCP and TFRC when both flows use the same packet size. For example,
a packet drop rate of 0.1 means that 10% of the TCP and TFRC packets
are dropped. The TFRC flow is configured to send at most 100 packets
per second. There is good relative fairness until the packet drop
percentages reach 40 and 50%, when the TFRC flow receives three to
five times more bandwidth than the standard TCP flow. We note that
an ECN TCP flow would receive a higher throughput than the TFRC flow
at these high packet drop rates, if ECN-marking was still being used
instead of packet dropping. However, we don't use the ECN TCP
sending rate in these high-packet-drop scenarios as the target
sending rate for TFRC, as routers are advised to drop rather than
mark packets during high levels of congestion (Section 7 of
[RFC3168]). In addition, there is likely to be buffer overflow in
scenarios with such high packet dropping/marking rates, forcing
routers to drop packets instead of ECN-marking them.
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RFC 4828 TFRC: The SP Variant April 2007
TCP Pkt TFRC Pkt
Byte DropRate DropRate TCP/TFRC
DropRate (1460B seg) (14B seg) Pkt Drop Ratio
-------- ----------- --------- --------------
0.00001 0.015 0.0006 26.59
0.0001 0.13 0.0056 24.94
0.001 0.77 0.054 14.26
0.005 0.99 0.24 4.08
0.01 1.00 0.43 2.32
0.05 1.00 0.94 1.05
Table 11: Packet Drop Rate Ratio vs. Byte Drop Rate
Table 11 converts the byte drop rate p to packet drop rates for the
TCP and TFRC packets, where the packet drop rate for an N-byte packet
is 1-(1-p)^N. Thus, a byte drop rate of 0.001, with each byte being
dropped with probability 0.001, converts to a packet drop rate of
0.77, or 77%, for the 1500-byte TCP packets, and a packet drop rate
of 0.054, or 5.4%, for the 56-byte TFRC packets.
The right column of Table 11 shows the ratio between the TCP packet
drop rate and the TFRC packet drop rate. For low byte drop rates,
this ratio is close to 26.8, the ratio between the TCP and TFRC
packet sizes. For high byte drop rates, where even most small TFRC
packets are likely to be dropped, this drop ratio approaches 1. As
Table 10 shows, with byte drop rates, the Standard TFRC sending rate
is close to optimal, competing fairly with a TCP connection using the
optimal packet size within the allowed range. In contrast, the
TFRC-SP connection gets more than its share of the bandwidth, though
it does reduce its sending rate for a byte drop rate of 0.01 or more
(corresponding to a TFRC-SP *packet* drop rate of 0.43.
Table 10 essentially shows three separate regions. In the low-
congestion region, with byte drop rates less than 0.0001, the TFRC-SP
connection is sending at its maximum sending rate. In this region
the optimal TCP connection is the one with 1460-byte segments, and
the TCP sending rate goes as 1/sqrt(p), for packet drop rate p. This
low-congestion region holds for packet drop rates up to 10-15%.
In the middle region of Table 10, with byte drop rates from 0.0001 to
0.005, the optimal TCP segment size is a function of the byte drop
rate. In particular, the optimal TCP segment size seems to be the
one that keeps the packet drop rate at most 15%, keeping the TCP
connection in the regime controlled by a 1/sqrt(p) sending rate, for
packet drop rate p. For a TCP packet size of S bytes (including
headers), and a *byte* drop rate of B, the packet drop rate is
roughly B*S. For a given byte drop rate in this regime, if the
optimal TCP performance occurs with a packet size chosen to give a
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RFC 4828 TFRC: The SP Variant April 2007
packet drop rate of at most 15%, keeping the TCP connection out of
the regime of exponential backoffs of the retransmit timer, then this
requires B*S = 0.15, or S = 0.15/B.
In the high-congestion regime of Table 10, with high congestion and
with byte drop rates of 0.01 and higher, the TCP performance is
dominated by the exponential backoff of the retransmit timer
regardless of the segment size. Even a 40-byte packet with a zero-
byte data segment would have a packet drop rate of at least 33%. In
this regime, the optimal TCP *sending* rate comes with a large,
1460-byte data segment, but the TCP sending rate is low with any
segment size, considerably less than one packet per round-trip time.
We note that in this regime, while a larger packet gives a higher TCP
*sending* rate, a smaller packet gives a better *goodput* rate.
In general, Tables 8 and 9 show good performance for TFRC-SP in
environments with stable packet drop rates, where the decision to
drop a packet is independent of the packet size. However, in some
environments the packet size might affect the likelihood that a
packet is dropped. For example, with heavy congestion and a Drop
Tail queue with a fixed number of bytes rather than a fixed number of
packet-sized buffers, small packets might be more likely than large
packets to find room at the end of an almost-full queue. As a
further complication, in a scenario with Active Queue Management, the
AQM mechanism could either be in packet mode, dropping each packet
with equal probability, or in byte mode, dropping each byte with
equal probability. Sections B.3 and B.4 show simulations with
packets dropped at Drop-Tail or AQM queues, rather that from a
probabilistic process.
B.3. Packet Dropping Behavior at Routers with Drop-Tail Queues
One of the problems with comparing the throughput of two flows using
different packet sizes is that the packet size itself can influence
the packet drop rate [V00, WBL04].
The default TFRC was designed for rough fairness with TCP, for TFRC
and TCP flows with the same packet size and experiencing the same
packet drop rate. When the issue of fairness between flows with
different packets sizes is addressed, it matters whether the packet
drop rates experienced by the flows is related to the packet size.
That is, are small packets just as likely to be dropped as large TCP
packets, or are the smaller packets less likely to be dropped
[WBL04]? And what is the relationship between the packet-dropping
behavior of the path, and the loss event measurements of TFRC?
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RFC 4828 TFRC: The SP Variant April 2007
< - - - - - Send Rates in Kbps - - - - >
Web TCP (1460B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.04 316.18 0.05 183.05
25 0.07 227.47 0.07 181.23
50 0.08 181.10 0.08 178.32
100 0.14 85.97 0.12 151.42
200 0.17 61.20 0.14 73.88
400 0.20 27.79 0.18 36.81
800 0.29 3.50 0.27 16.33
1600 0.37 0.63 0.33 6.29
Table 12: Drop and Send Rates for Drop-Tail Queues in Packets
Table 12 shows the results of the second half of 100-second
simulations, with five TCP connections and five TFRC-SP connections
competing with web traffic in a topology with a 3 Mbps shared link.
The TFRC-SP application generates 200-byte data packets every 10 ms,
for a maximum data rate of 160 Kbps. The five long-lived TCP
connections use a data packet size of 1460 bytes, and the web traffic
uses a data packet size of 512 bytes. The five TCP connections have
round-trip times from 40 to 240 ms, and the five TFRC connections
have the same set of round-trip times. The SACK TCP connections in
these simulations use the default parameters in the NS simulator,
with Limited Transmit, and a minimum RTO of 200 ms. Adding
timestamps to the TCP connection didn't change the results
appreciably. The simulations include reverse-path traffic, to add
some small control packets to the forward path, and some queueing
delay to the reverse path. The number of web sessions is varied to
create different levels of congestion. The Drop-Tail queue is in
units of packets, which each packet arriving to the queue requires a
single buffer, regardless of the packet size.
Table 12 shows the average TCP and TFRC sending rates, each averaged
over the five flows. As expected, the TFRC-SP flows see similar
packet drop rates as the TCP flows, though the TFRC-SP flows receives
higher throughput than the TCP flows with packet drop rates of 25% or
higher.
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RFC 4828 TFRC: The SP Variant April 2007
< - - - - - Send Rates in Kbps - - - - >
Web TCP (1460B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.061 239.81 0.004 185.19
25 0.089 189.02 0.006 184.95
50 0.141 99.46 0.013 185.07
100 0.196 16.42 0.022 183.77
200 0.256 4.46 0.032 181.98
400 0.291 4.61 0.051 151.88
800 0.487 1.01 0.078 113.10
1600 0.648 0.67 0.121 65.17
Table 13: Drop and Send Rates for Drop-Tail Queues in Bytes I:
1460B TCP Segments
However, the fairness results can change significantly if the Drop-
Tail queue at the congested output link is in units of bytes rather
than packets. For a queue in packets, the queue has a fixed number
of buffers, and each buffer can hold exactly one packet, regardless
of its size in bytes. For a queue in bytes, the queue has a fixed
number of *bytes*, and an almost-full queue might have room for a
small packet but not for a large one. Thus, for a simulation with a
Drop-Tail queue in bytes, large packets are more likely to be dropped
than are small ones. The NS simulator doesn't yet have a more
realistic intermediate model, where the queue has a fixed number of
buffers, each buffer has a fixed number of bytes, and each packet
would require one or more free buffers. In this model, a small
packet would use one buffer, while a larger packet would require
several buffers.
The scenarios in Table 13 are identical to those in Table 12, except
that the Drop-Tail queue is in units of bytes instead of packets.
Thus, five TCP connections and five TFRC-SP connections compete with
web traffic in a topology with a 3 Mbps shared link, with each TFRC-
SP application generating 200-byte data packets every 10 ms, for a
maximum data rate of 160 Kbps. The number of web sessions is varied
to create different levels of congestion. However, instead of Drop-
Tail queues able to accommodate at most a hundred packets, the
routers' Drop-Tail queues are each able to accommodate at most 50,000
bytes (computed in NS as a hundred packets times the mean packet size
of 500 bytes).
As Table 13 shows, with a Drop-Tail queue in bytes, the TFRC-SP flow
sees a much smaller packet drop rate than the TCP flow, and as a
consequence receives a much larger sending rate. For the simulations
in Table 13, the TFRC-SP flows use 200-byte data segments, while the
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RFC 4828 TFRC: The SP Variant April 2007
long-lived TCP flows use 1460-byte data segments. For example, when
the five TCP flows and five TFRC-SP flows share the link with 800 web
sessions, the five TCP flows see an average drop rate of 49% in the
second half of the simulation, while the five TFRC-SP flows receive
an average drop rate of 8%, and as a consequence receive more than
100 times the throughput of the TCP flows. This raises serious
questions about making the assumption that flows with small packets
see the same packet drop rate as flows with larger packets. Further
work will have to include an investigation into the range of
realistic Internet scenarios, in terms of whether large packets are
considerably more likely to be dropped than are small ones.
< - - - - - Send Rates in Kbps - - - - >
Web TCP (512B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.02 335.05 0.00 185.16
25 0.02 289.94 0.00 185.36
50 0.04 139.99 0.01 184.98
100 0.06 53.50 0.01 184.66
200 0.10 16.14 0.04 167.87
400 0.16 6.36 0.07 114.85
800 0.24 0.90 0.11 67.23
1600 0.42 0.35 0.18 39.32
Table 14: Drop and Send Rates for Drop-Tail Queues in Bytes II:
512B TCP Segments
Table 14 shows that, in these scenarios, the long-lived TCP flows
receive a higher packet drop rate than the TFRC-SP flows, and receive
considerably less throughput, even when the long-lived TCP flows use
512-byte segments.
To show the potential negative effect of TFRC-SP in such an
environment, we consider a simulation with N TCP flows, N TFRC-SP
flows, and 10*N web sessions, for N ranging from 1 to 50, so that the
demand increases from all types of traffic, with routers with Drop-
Tail queues in bytes.
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RFC 4828 TFRC: The SP Variant April 2007
starved, while the TFRC-SP flows each continue to send 57 Kbps. In
contrast, when standard TFRC is used instead of TFRC-SP for the flows
sending 200-byte segments, the TCP flows are not starved (although
they still don't receive their "share" of the link bandwidth when
their packet drop rates are 30% or higher). These two sets of
simulations illustrate the dangers of a widespread deployment of
TFRC-SP in an environment where small packets are less likely to be
dropped than large ones.
B.4. Packet-dropping Behavior at Routers with AQM
As expected, the packet-dropping behavior also can be varied by
varying the Active Queue Management (AQM) mechanism in the router.
When the routers use RED (Random Early Detection), there are several
parameters than can affect the packet-dropping or marking behavior as
a function of the packet size.
First, as with Drop-Tail, the RED queue can be in units of either
packets or bytes. This can affect the packet-dropping behavior when
RED is unable to control the average queue size, and the queue
overflows.
Second, and orthogonally, RED can be configured to be either in
packet mode or in byte mode. In packet mode, each *packet* has the
same probability of being dropped by RED, while in byte mode, each
*byte* has the same probability of being dropped. In packet mode,
large-packet and small-packet flows receive roughly the same packet
drop rate, while in byte mode, large-packet and small-packet flows
with the same throughput in bps receive roughly the same *number* of
packet drops. [EA03] assessed the impact of byte vs. packet modes on
RED performance.
The simulations reported below show that for RED in packet mode, the
packet drop rates for the TFRC-SP flows are similar to those for the
TCP flows, with a resulting acceptable throughput for the TFRC-SP
flows. This is true with the queue in packets or in bytes, and with
or without Adaptive RED (discussed below). As we show below, this
fairness between TCP and TFRC-SP flows does not hold for RED in byte
mode.
The third RED parameter that affects the packet-dropping or marking
behavior as a function of packet size is whether the RED mechanism is
using Standard RED or Adaptive RED; Adaptive RED tries to maintain
the same average queue size, regardless of the packet drop rate. The
use of Adaptive RED allows the RED mechanism to function more
effectively in the presence of high packet drop rates (e.g., greater
than 10%). Without Adaptive RED, there is a fixed dropping
threshold, and all arriving packets are dropped when the dropping or
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RFC 4828 TFRC: The SP Variant April 2007
marking rate exceeds this threshold. In contrast, with Adaptive RED,
the dropping function is adapted to accommodate high-drop-rate
regimes. One consequence is that when byte mode is used with
Adaptive RED, the byte mode extends even to high-drop-rate regimes.
When byte mode is used with standard RED, however, the byte mode is
no longer in use when the drop rate exceeds the fixed dropping
threshold (set by default to 10% in the NS simulator).
In the simulations in this section, we explore the TFRC-SP behavior
over some of this range of scenarios. In these simulations, as in
Section B.3 above, the application for the TFRC-SP flow uses 200-byte
data packets, generating 100 packets per second.
< - - - - - Send Rates in Kbps - - - - >
Web TCP (1460B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.05 305.76 0.04 182.82
25 0.06 224.16 0.06 175.91
50 0.09 159.12 0.08 152.51
100 0.13 90.77 0.11 106.13
200 0.14 48.53 0.14 70.25
400 0.20 22.08 0.19 41.50
800 0.27 3.55 0.25 17.50
1600 0.42 1.87 0.34 8.81
Table 17: Drop and Send Rates for RED Queues in Packet Mode
For the simulations in Table 17, with a congested router with a RED
queue in packet mode, the results are similar to those with a Drop-
Tail queue in packets, as in Table 12 above. The TFRC-SP flow
receives similar packet drop rates as the TCP flow, though it
receives higher throughput in the more congested environments. The
simulations are similar with a RED queue in packet mode with the
queue in bytes, and with or without Adaptive RED. In these
simulations, TFRC-SP gives roughly the desired performance.
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RFC 4828 TFRC: The SP Variant April 2007
packets and the 240-byte TFRC-SP packets. However, the sending rates
for TFRC-SP in the scenarios in Table 19 with higher packet drop
rates are still greater than desired, even given the goal of fairness
with TCP flows with 1500-byte data segments instead of 512-byte data
segments.
< - - - - - Send Rates in Kbps - - - - >
Web TCP (1460B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.04 318.10 0.02 185.34
25 0.08 175.34 0.03 184.38
50 0.10 81.60 0.04 181.95
100 0.12 28.51 0.05 178.79
200 0.20 3.65 0.06 173.78
400 0.27 1.44 0.08 161.41
800 0.40 0.58 0.06 159.62
1600 0.55 0.29 0.02 180.92
Table 20: Drop and Send Rates with Adaptive RED Queues in Byte Mode
For the simulations in Table 20, the congested router uses an
Adaptive RED queue in byte mode.
For this router, the output queue is in units of bytes rather than of
packets, and each *byte* is dropped with the same probability. Also,
because of the use of Adaptive RED, this byte-dropping mode extends
even for the high-packet-drop-rate regime.
As Table 20 shows, for a scenario with Adaptive RED in byte mode, the
packet drop rate for the TFRC-SP traffic is *much* lower than that
for the TCP traffic, and as a consequence, the sending rate for the
TFRC-SP traffic in a highly congested environment is *much* higher
than that of the TCP traffic. In fact, in these scenarios the TFRC-
SP congestion control mechanisms are largely ineffective for the
small-packet traffic.
The simulation with 1600 web servers is of particular concern,
because the TCP packet drop rate increases, while the TFRC-SP packet
drop rate decreases significantly. This is due to a detail of the
Adaptive RED implementation in the NS simulator, and not about the
dynamics of TFRC-SP. In particular, Adaptive RED is configured not
to "adapt" to packet drop rates over 50%. When the packet drop rate
is at most 50%, Adaptive RED is moderately successful in keeping the
packet drop rate proportional to the packet size. TCP packets are
six times larger than the TFRC-SP packets (including headers), so the
TCP packets should see a packet drop rate roughly six times larger.
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RFC 4828 TFRC: The SP Variant April 2007
But for packet drop rates over 50%, Adaptive RED is no longer in this
regime, so it is no longer successful in keeping the drop rate for
TCP packets at most six times the drop rate of the TFRC-SP packets.
We note that the unfairness in these simulations, in favor of TFRC-
SP, is even more severe than the unfairness shown in Table 13 for a
Drop-Tail queue in bytes. At the same time, it is not known if there
is any deployment in the Internet of any routers with Adaptive RED in
byte mode, or of any AQM mechanisms with similar behavior; we don't
know the extent of the deployment of standard RED, or of any of the
proposed AQM mechanisms.
< - - - - - Send Rates in Kbps - - - - >
Web TCP (512B seg) TFRC-SP (200B seg)
Sessions DropRate SendRate DropRate SendRate
-------- -------- -------- -------- --------
10 0.01 306.56 0.01 185.11
25 0.02 261.41 0.01 184.41
50 0.02 185.07 0.01 184.54
100 0.04 59.25 0.03 181.58
200 0.08 16.32 0.06 150.87
400 0.12 11.53 0.10 98.10
800 0.25 5.85 0.15 46.59
1600 0.32 1.43 0.22 19.40
Table 21: Drop and Send Rates for Adaptive RED Queues in Byte Mode
Table 21 shows that when TFRC-SP with 240-byte packets competes with
TCP with 552-byte packets in a scenario with Adaptive RED in byte
mode, the TFRC-SP flows still receive more bandwidth than the TCP
flows, but the level of unfairness is less severe, and the packet
drop rates of the TCP flows are at most twice that of the TFRC-SP
flows. That is, the TFRC-SP flows still receive more than their
share of the bandwidth, but the TFRC-SP congestion control is more
effective than that in Table 20 above.
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RFC 4828 TFRC: The SP Variant April 2007Appendix C. Exploring Possible Oscillations in the Loss Event Rate
TFRC-SP estimates the loss interval size differently for short and
long loss intervals, counting only one loss event for long loss
intervals, but counting all packet losses as loss events for the
short loss intervals. One question that has been raised is whether
this can lead to oscillations in the average loss event rate in
environments where there are many packet drops in a single loss
event, and loss events switch from short to long and vice versa. As
an example, consider a loss interval with N packets, including N/4
losses. If this loss interval is short (at most two round-trip
times), the loss interval length is measured as 4 packets. If this
loss interval is long, then the loss interval length is measured as N
packets.
If the loss interval switching from short to long and back leads to
severe oscillations in the average packet drop rate, and therefore in
the allowed sending rate, one solution would be to have a more
gradual transition between the calculation of the loss interval
length for short and long loss intervals. For example, one
possibility would be to use all of the packet losses for a loss
interval of one round-trip time in calculating the loss interval
length, to use 2/3 of the packet losses from a loss interval of two
round-trip times, to use 1/3 of the packet losses from a loss
interval of three round-trip time, and to use only one packet loss
from a loss interval of four or more round-trip times. This more
gradual mechanism would give a transition to counting all losses for
a loss interval of only one round-trip time, and counting only one
loss event for a loss interval of four or more round-trip times.
However, our simulations so far have not shown a great difference in
oscillations in the estimate loss event rate between the default
mechanism for estimating the loss interval length for short loss
interval and the gradual mechanism described above. Simulation
scripts are available from [VOIPSIMS]. Therefore, for now we are
staying with the simple default mechanism for estimating the loss
event rate for short loss intervals described in this document.
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RFC 4828 TFRC: The SP Variant April 2007Appendix D. A Discussion of Packet Size and Packet Dropping
The lists below give a general summary of the relative advantages of
packet-dropping behavior at routers independent of packet size,
versus packet-dropping behavior where large packets are more likely
to be dropped than small ones.
Advantages of Packet Dropping Independent of Packet Size:
1. Adds another incentive for end nodes to use large packets.
2. Matches an environment with a limitation in pps rather than bps.
Advantages of Packet Dropping as a Function of Packet Size:
1. Small control packets are less likely to be dropped than are
large data packets, improving TCP performance.
2. Matches an environment with a limitation in bps rather than pps.
3. Reduces the penalty of TCP and other transport protocols against
flows with small packets (where the allowed sending rate is
roughly a linear function of packet size).
4. A queue limited in bytes is better than a queue limited in
packets for matching the worst-case queue size to the worst-case
queueing delay in seconds.
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RFC 4828 TFRC: The SP Variant April 2007
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